1. Field of the Invention
The present invention relates to copiers having automatic document feeders which produce production runs of multi-sheet documents.
2. Description of the Prior Art
The term contrast, as used herein, refers to the rate of change (or slope) of the output copy density Dout with respect to the input copy density Din.
High-speed copiers having automatic feeders are currently in use to produce production runs. In one example, a recirculating feeder is coupled to a copier and causes the copier to make collated copies of a multi-sheet document. The recirculating feeder automatically feeds individual sheets in succession from the bottom of a document stack (a multi-sheet document in the feeder tray) to the exposure platen and returns such sheet to the top of the document stack while maintaining the original orientation. After each sheet of a document stack has been fed once, the document stack can either be fed again or removed from the feeder. Since the copy sheets are delivered from the copier in the same order as the original document sheets, collation of the production run copy sheets by a sorter accessory is unnecessary.
Occasionally, a sheet of the document stack will have a contrast or density which when copied by the copier in normal operation will produce a poor copy. In this case, the practice is to remove this document sheet from the document stack and copy it individually, making an intermediate or master copy. The copy master is then returned to the appropriate position in the document stack and the production run undertaken. A problem with this arrangement is that during the production run when the copy master is copied, (making a copy of a copy), image quality may be unavoidably degraded.
In order to make the copy master, several techniques may be used to adjust the electrophotographic process to provide the master copy with improved contrast and density.
In one technique for making copy masters, halftone screens, including halftone tint screens, are used to improve the contrast and density of certain difficult-to-reproduce images. Halftone tint screens have opaque dots of uniform density. Typical screening techniques for producing half-toned copies of continuous images or of large image areas of uniform density generally involve inserting a screen in the optical path which transforms the image into a plurality of dots or lines which can then be developed. Such discrete charge bearing zones in the form of dots, lines or other subdivisions of such images hereinafter referred to as "charge islands", are separated by "open areas" bearing little or no charge relative to the charge islands. Electrostatic charge images comprising such charge islands also can be created by initially charging the electrographic surface in a screen pattern, by masking the original image with a halftone screen during image exposure, or by uniformly exposing a charged photoconductive surface through a halftone screen before, during, or after image exposure, but before development. For examples of such screening techniques, see U.S. Pat. Nos. 2,598,732 and 3,724,944.
In the above-identified commonly assigned patent application to Kasper et al, a halftone screen is included as an integral part of the photoconductive element. This arrangement offers several advantages including (1) minimizing registration problems and use of simple continuous exposure techniques since the screen moves with the photoconductive layer, (2) a fixed space is maintained between the screen pattern and the photoconductive layer and, (3) high frequency screen patterns may be used without significant resolution loss.
Another contrast and density technique is set forth in commonly assigned U.S. patent application Ser. No. 137,149, entitled Copy Contrast and Density Control, filed Apr. 4, 1980 to Fiske et al. The disclosed apparatus adjusts copier process parameters to produce copies having improved copy contrast and density. A memory has a stored matrix array of sets, with each set having values which correspond to specific levels of Vo, Eo, and V.sub.B respectively. The operator designates a particular set. Means responsive to the values of the designated set change the exposure Eo produced by exposure lamps, the voltage Vo applied onto the surface of a photoconductor by a charger and the bias V.sub.B applied to an electrode of a development station to provide a copy having improved contrast and density.